1. Field of the Invention
The present invention relates to distance measuring equipment (DME) that measures the distance between an aircraft and a ground apparatus. More particularly, the invention relates to a DME ground apparatus that is provided on the ground.
2. Description of the Related Art
The distance measuring equipment (herein after referred to as DME apparatus) is a secondary radar system composed of an airborne apparatus and a ground apparatus. The airborne apparatus is mounted in an aircraft. The ground apparatus is provided on the ground and communicates with the airborne apparatus.
The airborne apparatus (also known as DME airborne apparatus) has an interrogator. The ground apparatus (also known as DME ground apparatus) has a transmitting-receiving device called a transponder.
The interrogator incorporated in the airborne apparatus transmits interrogation pulses of UHF band (pair pulses) toward the transponder provided in the ground apparatus. (The interrogator and the transponder keep communicating, with a frequency difference of 63 MHz between them. This is because the frequency allocated to the interrogator is 1,025 to 1,150 MHz and the frequency allocated to the transponder is 962 to 1,213 MHz.) The distance between the aircraft and the ground apparatus is measured from the time that elapses until the interrogator receives response pulses (pair pulses) from the transponder after it has transmitted the interrogation pulses. (See, for example, Japanese Patent No. 2,629,612.)
The ground apparatus keeps transmitting random pulse signals at the rate of about 1,000 pps (Pulse pairs Per Second) even while no interrogation pulses are coming from the airborne apparatus. Only when the ground apparatus receives interrogation pulse signals from the airborne apparatus, it transmits response pulse signals in place of the random pulse signals. Therefore, pulse signals, including the random pulse signals and response pulse signals, are always transmitted from the ground apparatus at the rate of 1,000 to 2,700 pps, as long as the distance measuring equipment operates in normal state.
That is, the airborne apparatus sequentially transmits interrogation pulses to the ground apparatus, at random intervals (though the number of pulses per second is fixed, e.g., 30 pulses per second). The ground apparatus receives the interrogation pulse signal of a prescribed frequency from the airborne apparatus. The ground apparatus demodulates and decodes the interrogation pulse signal, and imparts a preset system delay time (e.g., 50 μs) to the interrogation pulse signal thus decoded. The ground apparatus then encodes the interrogation pulse signal, generating a response pulse signal. The response pulse signal is transmitted to the aircraft via a specific transmitting system.
In the aircraft, the airborne apparatus receives the response pulse signal and decodes the same. The airborne apparatus then measures the time that has elapsed from the transmission of the interrogation pulse signal to the reception of the response pulse signal. Since the speed with which the electric wave travels is fixed, the airborne apparatus calculates the distance between it and the ground apparatus from the time measured, using a prescribed calculation formula.
The ground apparatus can respond to the interrogations made in a plurality of airborne apparatuses (more precisely, the interrogators provided in the airborne apparatuses). The ground apparatus can therefore give about 100 aircrafts the data from which to calculate distances. Nonetheless, the ground apparatus neglects weak electric waves coming from far-off aircrafts if about 100 or more aircrafts make an access to it. Thus, the ground apparatus would not be over-loaded.
That is, in the ground apparatus, if the number of pulses transmitted increases too much, the amount of data the ground apparatus needs to stably receive the pulses increases. Consequently, in some cases, the ground apparatus may fail to respond, at a sufficiently high reliability, to any aircraft that is located near the ground apparatus and therefore needs to have distance data. To prevent this, an upper limit is set to the pulse transmission rate.
If the number of aircrafts using the ground apparatus increases, and the number of interrogation pulses from the airborne apparatus of the aircraft increases, the pulse transmission rate for the ground apparatus to respond to the interrogation pulse increases accordingly.
As the number of interrogation pulses increases, the pulse transmission rate may exceed the upper limit at the ground apparatus. In this case, the receiving sensitivity of the ground apparatus is lowered by using the automatic gain-control (AGC) function of the analog receiving system so as to reduce the number of interrogation pulses. The ground apparatus is thereby disabled from receiving the weak signals coming from the aircrafts located relatively far from the ground apparatus. The number of pulses received, which should be processed, is thereby reduced to achieve a control.
If a pulse receiving device incorporated in the ground apparatus is gain-controlled and therefore abruptly receives a high-level signal, an output circuit, detector and other circuits of the pulse receiving device may fail to preserve sufficient linearity. Even in this case, the blocking phenomenon is fast eliminated and the stable gain control can be achieved, as is reported in, for example, Japanese Patent No. 2,629,612. (Blocking phenomenon takes place when the pulse receiving device abruptly receives a high-level signal. Once it occurs, the gain control system cannot control the gain of the signal-amplifying system, saturating the waveform of the output signal of the receiving device, and the encoded pulses cannot be detected.)
As described above, the amount of data the DME ground apparatus needs to stably receive the pulses increases if the number of pulses transmitted from it increases. Consequently, the ground apparatus may fail to respond, at a sufficiently high reliability, to any aircraft that is located near the ground apparatus and therefore needs to have distance data. This is why an upper limit is set to the pulse transmission rate. As the number of interrogation pulses coming from the airborne apparatuses increases, the pulse transmission rate may exceed the upper limit at the ground apparatus. In this case, the receiving sensitivity of the ground apparatus is lowered to reduce the number of interrogation pulses by using the automatic gain-control (AGC) function of the analog receiving system. The ground apparatus is thereby disabled to receive the weak signals coming from the aircrafts flying relatively far from the ground apparatus. The number of pulses to process is thereby reduced, controlling the response reliability.
In the conventional DME ground apparatus, an analog circuit performs the process of reducing the number of pulses. Inevitably, it takes much time to finish this process, and the ground apparatus cannot be reduced in terms of circuit scale.
Japanese Patent No. 2,629,612 indeed describes the automatic gain control performed in a DME ground apparatus, but is silent about any process that should be performed if the number of pulses to transmit increases.
In view of the foregoing, the present invention has been made to provide a DME ground apparatus that can be well adjusted to work even if the number of pulses to transmit increases and that can be miniaturized.